The present disclosure relates to bidirectional switches, and more particularly to bidirectional switches formed on conductive substrates and made of nitride semiconductor.
A bidirectional switch, which can conduct currents in both directions and can withstand both positive and negative voltages, is used as a main switch of a matrix converter capable of highly efficiently converting power, a main switch of a semiconductor relay, etc.
In a bidirectional switch, it is important to reduce switching loss caused by the product of a transient voltage and a transient current during switching, and conduction loss consumed by resistance of a semiconductor element itself at an on state (hereinafter referred to as “on-resistance”). However, where a bidirectional switch is formed of a silicon (Si) semiconductor element, the on-resistance is difficult to reduce due to the material limit of Si.
In order to overcome the material limit and reduce the conduction loss, employment of a semiconductor element made of nitride semiconductor represented by GaN or wide-gap semiconductor such as silicon carbide (SiC) is being considered. Wide-gap semiconductor has a higher breakdown field than Si by about one order of magnitude. In particular, charge is generated at the heterojunction interface between aluminum gallium nitride (AlGaN) and gallium nitride (GaN) due to spontaneous polarization and piezoelectric polarization. As a result, a two-dimensional electron gas (2DEG) layer is formed, which has a sheet carrier concentration of 1×1013 cm−2 or more and high mobility of 1000 cm−2 V/sec or more even when it is undoped. Therefore, an AlGaN/GaN heterojunction field effect transistor (AlGaN/GaN-HFET) is expected as a power switching transistor with low on-resistance and a high breakdown voltage.
In particular, when an AlGaN/GaN-HFET has a dual gate structure, a bidirectional switch can be formed of a single semiconductor element (see for example, United States Patent Publication No. 2005/0189561). A dual-gate HFET is equivalent to two transistors coupled in series in opposite directions, and controls both of a current flowing from a first ohmic electrode to a second ohmic electrode, and a current flowing from the second ohmic electrode to the first ohmic electrode. Therefore, a dual-gate HFET can be miniaturized as compared to a conventional bidirectional switch formed by combining a plurality of power transistors such as power metal oxide semiconductor field effect transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs), etc. In addition, a dual-gate structure is advantageous in reducing on-resistance as compared to the case where a bidirectional switch includes two MOSFETs coupled in series.
Furthermore, MOSFETs and IGBTs generally have low reverse breakdown voltages. Thus, where a bidirectional switch is provided using, for example, IGBTs, two IGBTs are coupled in parallel in opposite directions, and a diode needs to be coupled to each of the IGBTs in series. This is also applicable to power MOSFETs. However, a dual-gate HFET made of wide-gap semiconductor does not require any diode, since it has a high reverse breakdown voltage. Therefore, no loss is caused by on-resistance of a diode.